A noise cancellation microphone system is gaining more importance nowadays, especially with the development of multimedia applications and wireless communication technologies. Although various solutions were proposed to enhance desired signal extraction, particularly desired speech, in noisy environments, there are still rooms for improvement in order to obtain a high Signal-to-Noise (SNR) ratio using very few microphones.
Various methods were commonly used to increase the SNR of desired speech signal. In a known speech enhancement method, a single microphone is used to pick up the desired speech signal with noise. The noise spectrum is estimated and subtracted from the speech signal (containing the noise) picked up by the microphone. In this way, the desired speech signal is separated from the noise. However this method is only effective with stationary noise, and also introduces high distortion to the desired speech signal.
Another known noise cancellation method uses two microphones, with one microphone located near the source of the desired signal, and another microphone located near the noise source. Thus, the signal picked up by the microphone arranged near the noise source can be used to adaptively cancel the noise signal contained in the signal picked up by the microphone located near the desired speech signal. However, this method is not practical in most applications as it is very difficult to arrange a microphone near the noise source.
A further known microphone array system uses more than two microphones. The system uses spatial and temporal filtering method to enhance the desired speech signal from a specific direction and over an interested frequency band, and suppress any other signals from other directions. It can enhance the desired signal with a high SNR improvement. However the use of more than two microphones results in a large size of the system, making many mobile applications unsuitable. In the system disclosed in [1], more than three microphones are used to form a uni-directional microphone system for noise cancellation. Since in this system, there are no adaptive signal processing method used, the spatial response of this microphone system is fixed. This makes the whole system inflexible and also results in the performance of noise/interference cancellation to be poor.
Accordingly to the system described in [2], two microphones are used to form a first-order microphone system for noise cancellation. However, this microphone system uses only the differential property of sound field to form a fixed beam pattern, the performance of the system is therefore poor, especially in environments with complicated noise signals.
In the system disclosed in [3], a cardioid-type directional microphone and an omni-directional microphone are combined in an acoustically coupled way. The two microphones, together with an adaptive control circuit, produces a very narrow 3-dimensional beam for acquiring the desired speech signal. However the adaptive filter in the adaptive control circuit uses a normal Least-Mean-Squared (LMS) algorithm with a fixed step size or an adaptive step size which is based on the signal correlation coefficient. Such a method gives rise to performance degradation due to wrong updating of the coefficients of the adaptive filter when speech and noise signal are present simultaneously. This results in low noise suppression and high desired signal cancellation.
Therefore, a noise cancellation microphone system with a high SNR improvement, but is compact in size is desired.